1. Field of the Invention
This invention relates to an intake system for a supercharged engine in which a supercharger and an intercooler are disposed in an air intake passage and, more particularly, to an intake system, for a supercharged engine, which is provided with a recirculation path and a recirculation control valve for recirculating supercharging air when the engine is operating under low engine loads.
2. Description of the Related Art
An air intake system is known in which a mechanical supercharger, driven by the engine output shaft, is provided, along with a recirculation control valve and a relief path, in an air intake path so as to allow supercharging air to be recirculated under low load engine operating conditions. Such a control system is known from, for instance, Japanese Unexamined Patent Publication No. 2-283816.
The layout of the intake system described in this publication disposes a mechanical supercharger downstream from a throttle valve of an air intake path or passage and an intercooler downstream from the supercharger for cooling supercharging air. A recirculation path or passage, i.e. a supercharging air bypass passage, is provided between the supercharger and the intercooler, in which path there is provided a recirculation control valve. The recirculation control valve, which is directly connected with and operated by a pressure controlled actuator, into which pressure downstream from the throttle valve is introduced, opens the recirculation passage in a low engine load range in which the pressure downstream from the throttle valve is lower and closes it when the pressure downstream from the throttle valve increases with an increase in engine load.
With this type of an intake system of a supercharged engine, an energy loss in the supercharger becomes larger with an increase in ratio between pressures upstream and downstream from the supercharger. Accordingly, in order to prevent such an energy loss, it is desirable to recirculate part of air released from the supercharger while the engine is supplied with a sufficient amount of air according to engine operating conditions in a range of certain high engine loads during operation of the supercharger. In addition, in order to maintain a linear relationship between throttle opening and engine output torque, the recirculation control valve is desired to gradually decrease its opening with a change in throttle opening in a specific range. For these reasons, the recirculation control valve of the publication mentioned above is designed and adapted to gradually close with an increase in boost pressure downstream from the supercharger. However, if the recirculation control valve simply closes gradually with an increase in boost pressure, it causes hunting near engine operating conditions in which it fully closes due to a pressure drop caused by suspension of recirculation. As a result, the recirculation control valve is apt to be unstable in operation, and a range of engine operating conditions in which the recirculation control valve is fully closed is made unstable. In order to eliminate these adverse effects, it was thought to close rapidly the recirculation control valve when the recirculation control valve reaches a certain opening. However, such a rapid closing of the recirculation control valve causes torque shock.
Further, with this type of an intake system, when the recirculation control valve is open while the supercharger is operating, after the air being expelled from the supercharger passes through the intercooler, a necessary amount of the cooled air is delivered into combustion chambers of the engine, and an excess of the cooled air is recirculated to the upstream of the supercharger. As a consequence, in a range in which a small amount of air is taken, the air tends to be over-cooled, and the pressure of air between the supercharger and the intercooler is insufficiently lowered due to the resistance of air flow in the intercooler, resulting in a loss of energy.
In order to eliminate such energy loss, in addition to the provision of a recirculation passage communicating portions of the intake passage upstream and downstream from the supercharger, a bypass passage may be established downstream from the supercharger so as to allow discharged air to be delivered directly to the engine bypassing the intercooler when the engine operates under low engine loads. However, the provision of a bypass passage causes a requirement of controlling of air flowing through the recirculation passage in accordance with engine operating conditions so as to meet demands of engine output and fuel economy. In addition, in a range of low engine loads, in which a lesser amount of air is required, over-cooling must be prevented. On the other hand, in a range in which a relatively large amount of air is required while the recirculation passage is required to be open, a temperature increase caused by recirculation must be suppressed or controlled to be small. These requirements can not be satisfied by simply opening and closing of the bypass passage in the same manner as the recirculation passage.